The present invention relates to a bending unit of a machine for bending metal sheets.
The use is known of bending machines to deform a sheet in a permanent manner from planar to dihedral.
At the end of a well-executed bending operation, the resulting dihedron has planar faces inclined one with respect to the other by the amount required, that is the corner is rectilinear and the angles of all cross-sections perpendicular to the corner are equal to the required angle.
According to the prior art, a first type of bending unit of a bending machine includes two parts: a counterblade constituted by a parallelepiped bar having a length at least equal to the bend to be executed, on one of whose faces there is a V-shaped groove, and a blade also constituted by a parallelepiped bar having a length at least equal to the bend to be executed, one of whose faces is made into a convex V shape. The blade and counterblade, mounted on a single bending machine so that the two V-shaped corners, one concave and one convex, are parallel to one another, are brought together with a suitable force in the rectilinear direction which would bring the corners to coincide, if between them there were not interposed the metal sheet to be bent. The strip of metal sheet between the two edges of the V-shaped groove is thus subjected to a bending moment, at its greatest along the vertex of the convex V, which causes permanent deformation of the metal sheet at a position corresponding to the V-shaped vertex and the formation of a dihedron that much more acute the deeper the blade penetrates into the counterblade.
A second and a third type of bending unit of a bending machine each include three parts, two of which, a counterblade and a metal sheet holder, when they are compressed one against the other, hold one of the edges of the metal sheet adjacent to the future bending corner and leave the other edge free. The third part, the blade, has the function, by moving with respect to the other two and by interfering with the free edge, of deforming the metal sheet.
The second and third type of bending unit differ one from the other in the nature of the motion of the blade in relation to the counterblade and to the metal sheet holder. In the second type of bending unit, the blade has a planar face substantially coincident with a face of the edge to be bent and, in order to execute the bend, this blade rotates with a suitable torque around a straight line which approximately coincides with the corner of the final bend. In the third type of bending unit, the blade touches the edge to be bent along a straight line parallel to a bending corner with a cylindrical face and, in order to execute the bend, the blade translates with a suitable force along a curvilinear trajectory, in particular a rectilinear one, lying in a plane perpendicular to the bending corner.
With the three known bending units, even in the event that the blade moves in an ideal manner, it is not always possible to ensure that a bend will be produced with a rectilinear corner and with the angle constant along its entire length, independently of the thickness and of the length of the bent sheet.
In fact, each part of the bending unit is subjected to forces which vary in the areas facing the metal sheet and to no forces at all in the others. The materials of the areas not subjected to force contribute to the rigidity of the part in the nearby areas under stress more than to that in the far areas under stress. It follows that the bend is more closed in the areas in which the part is more rigid and more open in those in which the part is more flexible, because they are far from those not under stress.
To allow the execution with the same bending machine of several parallel bends on the same metal sheet, even if they are very close to one another, the three parts of the bending unit should have a minimum size sufficient to prevent fatigue breakage under the forces caused by the bending operation. In actual fact, if the sizes of the cross-sections perpendicular to the bending corners of those three parts of the bending unit were to be reduced to the minimum compatible with the resistance, the difference between their deformations in the central part of the bend and in the peripheral areas, which are under the effect of the support of the areas external to the bend, would be so large as to induce unacceptable variations in the bending angle along the bending corner.
In practice, a compromise has had to be adopted up to now, different according to the use of the bent metal sheet pieces, between giving-up the execution of many close-up bends on the same sheet and giving-up a constant bending angle.